1. Field of the Invention
The present invention relates to power converters, and more specifically relates to the control of switching power converters.
2. Description of Related Art
Switching power converters have been widely used to provide regulated voltage and current. A transformer (an inductive device) is used in the power converter for energy store and power transfer. FIG. 1 shows a circuit schematic of a power converter. A controller 15 generates a switching signal SW at an output terminal OUT to regulate the output of the power converter in response to a feedback signal VFB. In general, the feedback signal VFB is obtained at a feedback terminal FB of the controller 15 by detecting the output voltage VO of the power converter through an optical-coupler or a feedback circuit including an auxiliary winding (Figure not shown).
The switching signal SW drives a power transistor 12 for switching a transformer 10. The transformer 10 is connected to an input voltage VIN of the power converter. The energy of the transformer 10 is transferred to the output voltage VO of the power converter through a rectifier 17 and a capacitor 18. A resistor RS is connected serially with the power transistor 12 to generate a current signal VI in response to a switching current IP of the transformer 10. The current signal VI is coupled to a current-sense terminal VS of the controller 15 for the control and protections of the power converter. A resistor 19 is further connected from the input voltage VIN to an input terminal IN of the controller 15 for over-voltage and under-voltage protections, etc.
Furthermore, the over-power protection of power converter requires sensing the input voltage VIN to control the maximum output power as a constant. The approach was disclosed as “PWM controller for controlling output power limit of a power supply” by Yang et al., U.S. Pat. No. 6,611,439. The drawback of this prior art is the power loss caused by the resistor 19 especially when the input voltage VIN is high. The object of the present invention is to sense the input voltage VIN without the need of the resistor 19 for saving power. Moreover, reducing input terminals of the controller 15 is another object of the present invention.